There has been known that when, to an outermost layer of a transparent substrate, a low refractive index layer (a reflection-reducing layer) that is made from a material having a lower refractive index than the substrate is formed in a quarter thickness (approximately 100 nm) of a visible wavelength, there is a reduction in surface reflectance. An antireflection transparent substrate made of a film or glass to which this principle is applied has been in widespread use in a variety of fields, such as electronic products, optical products, building materials, and the like.
As a method for forming the reflection-reducing layer, a dry coating method with which magnesium fluoride or the like, is vapor-deposited or sputtered, and a wet coating method with which a solution of a low refractive index material is applied to a substrate have been known. Of the methods, the wet coating method has attracted attention recently, because it has little restrictions as to substrates it can adopt, and is advantageous in continuous production and costs over the other.
As the low refractive index material for the wet coating method, a material made from a combination of fluorine resin, porous silica, or hollow silica, and a film-forming matrix has been known (See Japanese Unexamined Patent Publication Nos. 85701/1988 (Tokukaisyo 63-85701) and 2001-233611 (Tokukai 2001-233611)).
Hollow silica having a particle diameter of approximately 0.1 μm to 300 μm, and a method for producing the hollow silica have been well known already (See Japanese Unexamined Patent Publication Nos. 258642/1988 (Tokukaisyo 63-258642) and 330606/1994 (Tokukaihei 6-330606)). Japanese Unexamined Patent Publication No. 330606/1994 (Tokukaihei 6-330606) discloses a technique with which (1) an organic solvent is added to and mixed with an inorganic material, such as a silicate, so as to make an oil-in-water (O/W) type emulsion; and then, (2) an organic solvent containing a lipophilic surfactant is added to and mixed with the emulsion so as to make an oil-in-water-in-oil (O/W/0) type emulsion, and finally, (3) an inorganic compound are turned into a water-insoluble deposit by use of an inorganic acid or an ammonium salt of the inorganic acid, so that inorganic hollow fine particles are obtained. Since the technique was disclosed, other various methods for producing the hollow silica particles still have been disclosed (See Japanese Unexamined Patent Publication Nos. 133105/1995 (Tokukaihei 7-133105), 2001-233611 (Tokukai 2001-233611), 29318/1999 (Tokukaihei 11-29318) and Japanese translation of PCT international publication No. 500113/2000 (Tokuhyo 2000-500113)).
Japanese Unexamined Patent Publication No. 133105/1995 (Tokukaihei 7-133105) discloses a method with which (1) a silicate of alkali metal, for example, and an alkali-soluble inorganic compound are turned into colloid particles by use of an alkaline solution having a pH of 10 or over, and then (2) elements of this particle, except silicone and oxygen, is partially removed, after that, (3) this particle is coated with a hydrolysable organosilicon compound or the like. Japanese Unexamined Patent Publication No. 29318/1999 (Tokukaihei 11-29318) discloses a method with which a tetraalkoxysilane that is solubilized in water is emulsified in an organic solvent by use of a surfactant, so that a hydrolysis and a condensation reaction occur. In this case, when a moisture content is high, micron-size hollow silica particles are produced. Further, Japanese translation of PCT international publication No. 500113/2000 (Tokuhyo 2000-500113) discloses a method for obtaining the hollow silica by: depositing activated silica from alkaline silicate metal on a core made from a material other than silica; and removing the core.
However, these techniques have a lot of problems, such as: a problem that the hollow silica easily breaks in a process due to a large number of pores in a silica layer or a thin thickness of the silica layer; a low porosity caused by significant remains of nuclear particles in hollows; limited usage resulting from the fact that the conventional methods can only realize the micron-size hollow silica having a wide particle size distribution; and low productivity due to a long reaction period and a large number of steps.